1. Field of the Invention
The present invention relates to a semiconductor memory device which uses variable-resistance elements in memory cells to write a binary digit by a current flowing through the variable-resistance element.
2. Description of the Related Art
In recent years, much attention has been paid to semiconductor memories using variable-resistance elements as memory elements, for example, a phase-change random access memory (PCRAM) and a magnetic random access memory (MRAM), and they have been developed. The MRAM is a device in which binary digits are stored in memory cells by using the magnetoresistive effect to perform a memory operation, and has combined features of non-volatility, high speed operation, high integration and high reliability. Therefore, an SRAM, a pseudo-SRAM (PSRAM), DRAM or the like is positioned as one of the candidates of replaceable memory devices.
A large number of MRAMs each using elements indicating the tunneling magnetoresistive (TMR) effect among the magnetoresistive effects have been reported. As the TMR effect element, a magnetic tunnel junction (MTJ) element is usually used which has a laminated structure constituted of two ferromagnetic layers and a nonmagnetic layer sandwiched between these layers and which uses the change of a magnetic resistance due to a spin polarization tunnel effect. The MTJ element can take a low-resistance state and a high-resistance state in accordance with the magnetization arrangement of the two ferromagnetic layers. The low-resistance state is defined as binary 0, and the high-resistance state is defined as binary 1, whereby one binary digit can be recorded in the MTJ element.
Writing to the MRAM can be accomplished by, for example, supplying a write current to the MTJ element, whereby in accordance with the direction of this write current, the magnetization arrangement of the MTJ elements is changed from a parallel state to an anti-parallel state, or from the anti-parallel state to the parallel state. For example, a usual 1Tr+1MTJ type memory cell takes a connecting constitution in which one terminal of the MTJ element is connected to a first bit line, the other terminal of the MTJ element is connected to one source/drain region of a select transistor, and the other source/drain region of the select transistor is connected to a second bit line.
There has been disclosed a technology where, in the MRAM having such a constitution, the connection between an active area formed under the first bit line and the second bit line is achieved by extending the active area (Jpn. Pat. Appln. KOKAI Publication No. 2008-192990). However, the active area has a high sheet resistance, and hence the parasitic resistance of the MTJ element increases. Consequently, the read margin might be reduced.